Fast automatic gain control circuit with adjustable range

ABSTRACT

An automatic gain control circuit has its a-c signal input terminal connected directly to the numerator input of an analog divider, and connected through a peak detector to the denominator input of the analog divider. The quotient terminal of the analog divider provides a gain-controlled a-c signal output. The AGC circuit produces a constant amplitude output signal so long as the amplitude of the input signal exceeds an adjustable reference voltage applied to the peak detector. The reference voltage may be changed to change the range of input voltages resulting in a constant-amplitude output. The AGC circuit responds almost instantly to control a sudden increase in input voltage.

BACKGROUND OF THE INVENTION

Although there are many known automatic gain control circuits, there isa need for an AGC circuit which responds very quickly to a suddenincrease in input voltage without over or under shoot, and whichproduces a constant output amplitude in response to input signalamplitudes within an adjustable range. One application for an AGCcircuit having the described characteristics is in an apparatus formeasuring the power of an internal combustion engine by performing anacceleration burst test without making electrical or mechanicalconnections to the engine.

SUMMARY OF THE INVENTION

An a-c input signal having a rapidly-increasing amplitude is translatedto a constant amplitude a-c output signal by dividing the input signalby the peak voltage of the input signal in an analog divider. The peakvoltage of the input signal is generated by a peak detector having areference voltage input which controls the range of input signalamplitudes resulting in a constant amplitude output signal.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a circuit diagram of an automatic gain control circuitconstructed according to the teachings of the invention;

FIG. 2 is an input-output characteristic chart which will be referred toin describing the operation of the invention; and

FIG. 3 is an input-output waveform chart which will be referred to indescribing the operation of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now in greater detail in FIG. 1, there is shown an automaticgain control circuit consisting of an a-c input signal terminal 10connected to the numerator input Z of an analog divider 12, andconnected through a peak detector 14 to the denominator input X of thedivider, and an output terminal 16 coupled to the quotient output 10Z/Xof the divider. The analog divider 12 may be an Intronics, Inc. typeD210 wide dynamic range analog divider. The output is equal to ten timesthe quotient of the numerator Z divided by the denominator X.

The peak detector 14 may include the two operational amplifiers 18 and20 of a Motorola Corp. operational amplifier unit MC1558. Theoperational amplifiers are connected with a diode 22, a resistor 24 anda capacitor 26 to form a noninverting peak detector. The operationalamplifier 20 is connected as a unity gain follower inside the overallfeedback loop. Every voltage peak of an a-c signal applied to the +input of operational amplifier 18 causes a corresponding charge to beadded to the charge or voltage on capacitor 26. This voltage istransmitted over lead 28 to the denominator input X of the analogdivider 12. A meter circuit 30 is provided for use if necessary whenmaking adjustments in the circuit for a particular input signal appliedto input terminal 10. Further information about the construction ofnoninverting peak detector circuits may be found on pages 355,356 of"Operational Amplifiers--Design and Applications" edited by Tobey,Graeme and Huelsman, and published by McGraw-Hill in 1971.

The peak detector 14 includes a resistor 32 and terminals 34 for theapplication of a d-c reference voltage from a source not shown. Areference voltage amplitude is selected so that all levels of inputsignal above a desired threshold (equal to the reference voltage) resultin a constant-amplitude output signal at the output terminal 16.

OPERATION

The input a-c signal applied to terminal 10 and to numerator input Z ofdivider 12 may be described by the formula:

    V.sub.p sin wt                                             (1)

where V_(p) is the peak voltage. This signal is also applied to the peakdetector, the output of which is equal to the voltage V_(p) when theinput signal peak voltage is greater than V_(ref). This voltage V_(p) isapplied to the denominator input X of the analog divider 12. The outputV_(o) at 16 from the divider is then: ##EQU1## The output remains at theconstant value of 10 sin wt regardless of variations in the value of theinput peak voltage V_(p).

On the other hand, if the peak voltage V_(p) of the input signal is lessthan the reference voltage V_(ref), the output at 28 from the peakdetector 14 is a voltage equal to V_(ref). Then the output V_(o) at 16from the analog divider 12 is: ##EQU2## which is a straight linerelationship of increasing voltage as V_(p) increases, because V_(ref)is constant.

FIG. 2 shows at 40 how the output peak voltage V_(o) varies in responseto input peak voltage when the reference voltage V_(ref) is equal to 8volts. The output is constant when the input peak voltage exceeds 8volts. The output peak voltage varies linearly with input peak voltagewhen input peak voltage is less than 8 volts.

The dashed curve 42 shows how the output peak voltage V_(o) voltagevaries in response to input peak voltage when the reference voltageV_(ref) is equal to 1 volt. The output is constant when the input peakvoltage exceeds 1 volt. It is thus seen that any desired dynamic rangeof input signal amplitudes over which a constant output signal amplitudeis provided can be established by merely employing a reference voltageV_(ref) of appropriate value.

FIG. 3 illustrates how rapidly the automatic gain control circuitoperates to limit the output signal amplitude when the input signalamplitude suddenly increases. The input wave A, having a frequency ofabout 100 Hz, has an initial low amplitude 48 which suddenly increasesduring a positive half cycle 50 of the input wave. The output wave B hasan initial period 52 during which the input wave is highly amplified,and then the amplification of the input half cycle 50 is immediatelylimited in the output wave at 54 to the predetermined gain controlledamplitude. Following cycles are also similarly limited. The wave shownby way of example increases in frequency because it is generated duringan acceleration burst test in an apparatus for measuring the power of aninternal combustion engine.

What is claimed is:
 1. An automatic gain control circuit having a signalinput terminal and a signal output terminal, comprisingan analog dividerhaving a numerator input coupled to said signal input terminal, andhaving a quotient output coupled to said signal output terminal, and apeak detector coupled from said signal input terminal to the divisorinput of said analog divider.
 2. A circuit as defined in claim 1 whereinsaid peak detector includes means to clamp its output at a referencevoltage when the signal input is less than the reference voltage.
 3. Acircuit as defined in claim 2 wherein said peak detector includes afirst operational amplifier having an output coupled through a diode toa capacitor and to a unity-gain follower buffer operational amplifierconnected in a feedback loop back to said first operational amplifier.4. A circuit as defined in claim 3 wherein a reference potential sourceis connected to charge said capacitor to said reference voltage.